Carbon Dioxide Recovery

ABSTRACT

The present techniques are directed to a system and method for recovering carbon dioxide (CO 2 ). The method includes recovering the CO 2  from a gas mixture including the CO 2  via a CO 2  separation system. The CO 2  separation system includes a rotating freezer/melter.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of U.S. Patent Application 61/775,164 filed Mar. 8, 2013 entitled CARBON DIOXIDE RECOVERY, the entirety of which is incorporated by reference herein.

FIELD OF THE INVENTION

The present disclosure relates generally to carbon dioxide (CO₂) recovery. More particularly, the present disclosure relates to systems and methods for recovering CO₂ from a gas mixture via a CO₂ separation system that includes a rotating freezer/melter.

BACKGROUND

This section is intended to introduce various aspects of the art, which may be associated with exemplary embodiments of the present techniques. This discussion is believed to assist in providing a framework to facilitate a better understanding of particular aspects of the present techniques. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art.

A conventional gas turbine engine often has a turbine compressor that is mechanically linked to an expander turbine through a shaft. The turbine compressor can be used to compress a flow of air ingested by the turbine compressor. The compressed air is then flowed to a combustor. In the combustor, fuel is injected and ignited to create a continuous flame. The high pressure exhaust gases from the flame are flowed into the expander turbine, which generates mechanical energy from the exhaust gas as it expands.

The exhaust gas may include a mixture of nitrogen (N₂), carbon dioxide (CO₂), water (H₂O), and any number of other gaseous components. In some cases, it may be desirable to remove at least a portion of the CO₂ from the exhaust gas as a CO₂ product. According to current techniques, solvent based separation processes, amine processes, pressure swing adsorption processes, or the like are used to recover the desired CO₂ product. However, the CO₂ product that is recovered using such processes is at a low pressure and must be compressed as a gas to a high pressure for use in enhanced oil recovery (EOR) or carbon storage applications.

One technique for removing CO₂ from the flue gas of a power station is described in U.S. Patent Application Publication No. 2011/0226010 by Baxter. Moisture is removed from the flue gas to yield a dried flue gas, and the dried flue gas is compressed to yield a compressed flue gas. The temperature of the compressed flue gas is then decreased using a first heat exchanger and a second heat exchanger. At least a portion of the CO₂ within the compressed flue gas condenses within the first and second heat exchangers, yielding a solid or liquid condensed-phase CO₂ component and a light-gas component. The condensed-phase CO₂ component can then be recovered. However, recovering the CO₂ product from the flue gas using such techniques may be costly due to the high degree of compression that is required.

SUMMARY

An exemplary embodiment of the present techniques provides a system for recovering carbon dioxide (CO₂). The system includes a CO₂ separation system configured to recover the CO₂ from a gas mixture. The CO₂ separation system includes a rotating freezer/melter.

Another exemplary embodiment provides a method for recovering carbon dioxide (CO₂). The method includes recovering the CO₂ from a gas mixture including the CO₂ via a CO₂ separation system. The CO₂ separation system includes a rotating freezer/melter.

Another exemplary embodiment provides a rotating freezer/melter for recovering carbon dioxide (CO₂) from a gas mixture. The rotating freezer/melter includes a freezing zone, a melting zone, and a rotor configured to rotate through the freezing zone and the melting zone. Solid CO₂ formed from a gas mixture is captured on the rotor while the rotor is rotating through the freezing zone, and the solid CO₂ melts and flows through the rotor as liquid CO₂ while the rotor is rotating through the melting zone.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the present techniques are better understood by referring to the following detailed description and the attached drawings, in which:

FIG. 1 is a block diagram of a system for power generation and carbon dioxide (CO₂) recovery;

FIG. 2 is a process flow diagram of a combined cycle power plant that can be used to produce electricity and generate a diluent gas mixture including CO₂;

FIG. 3 is a process flow diagram of a system for low emissions power generation and CO₂ recovery;

FIG. 4 is a process flow diagram of another system for low emissions power generation and CO₂ recovery;

FIG. 5 is a perspective view of a rotating freezer/melter that may be used to recover CO₂ from a gas mixture;

FIG. 6 is a cross-sectional view of the rotating freezer/melter that may be used to recover CO₂ from a gas mixture;

FIG. 7 is a perspective view of the rotor of the rotating freezer/melter of FIGS. 5 and 6;

FIG. 8 is a schematic showing flow paths within a section of the rotor of FIG. 7;

FIG. 9 is a perspective view of another rotor that may be used for the rotating freezer/melter of FIGS. 5 and 6;

FIG. 10 is a process flow diagram of a method for power generation and CO₂ recovery;

FIG. 11 is a generalized process flow diagram of a method for recovering CO₂ from a gas mixture; and

FIG. 12 is a block diagram of a system for recovering CO₂ from a natural gas stream.

DETAILED DESCRIPTION

In the following detailed description section, specific embodiments of the present techniques are described. However, to the extent that the following description is specific to a particular embodiment or a particular use of the present techniques, this is intended to be for exemplary purposes only and simply provides a description of the exemplary embodiments. Accordingly, the techniques are not limited to the specific embodiments described below, but rather, include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims.

At the outset, for ease of reference, certain terms used in this application and their meanings as used in this context are set forth. To the extent a term used herein is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in at least one printed publication or issued patent. Further, the present techniques are not limited by the usage of the terms shown below, as all equivalents, synonyms, new developments, and terms or techniques that serve the same or a similar purpose are considered to be within the scope of the present claims.

A “combined cycle power plant” is generally the combination of an open Brayton Cycle and a Rankine cycle. Combined cycle power plants typically use both steam and gas turbines to generate power, although other working fluids besides water and steam may be used in the Rankine cycle. The combined cycle gas/steam power plants generally have a higher energy conversion efficiency than gas or steam only plants. A combined cycle plant's efficiencies can be as high as 50% to 60% of a lower heating value (LHV). The higher combined cycle efficiencies result from synergistic utilization of a combination of the gas turbine with the steam turbine. Typically, combined cycle power plants utilize heat from the gas turbine exhaust to boil water to generate steam. The boilers in typical combined cycle plants can be referred to as heat recovery steam generator (HRSG). The steam generated is utilized to power a steam turbine in the combined cycle plant. The gas turbine and the steam turbine can be utilized to separately power independent generators, or in the alternative, the steam turbine can be combined with the gas turbine to jointly drive a single generator via a common drive shaft.

As used herein, a “compressor” includes any type of equipment designed to increase the pressure of a fluid or working fluid, and includes any one type or combination of similar or different types of compression equipment. A compressor may also include auxiliary equipment associated with the compressor, such as motors, and drive systems, among others. The compressor may utilize one or more compression stages, for example, in series. Illustrative compressors may include, but are not limited to, positive displacement types, such as reciprocating and rotary compressors for example, and dynamic types, such as centrifugal and axial flow compressors, for example. For example, a compressor may be a first stage in a gas turbine engine, as discussed in further detail below.

As used herein, “cooling” broadly refers to lowering and/or dropping a temperature and/or internal energy of a substance, such as by any suitable amount. Cooling may include a temperature drop of at least about 1 degree Celsius, at least about 5 degrees Celsius, at least about 10 degrees Celsius, at least about 15 degrees Celsius, at least about 25 degrees Celsius, at least about 50 degrees Celsius, at least about 100 degrees Celsius, and/or the like. The cooling may use any suitable heat sink, such as steam generation, hot water heating, cooling water, air, refrigerant, other process streams (integration), and combinations thereof. One or more sources of cooling may be combined and/or cascaded to reach a desired outlet temperature. The cooling step may use a cooling unit with any suitable device and/or equipment. According to one embodiment, cooling may include indirect heat exchange, such as with one or more heat exchangers. Heat exchangers may include any suitable design, such as shell and tube, plate and frame, counter current, concurrent, extended surface, and/or the like. In the alternative, the cooling may use evaporative (heat of vaporization) cooling and/or direct heat exchange, such as a liquid sprayed directly into a process stream.

“Cryogenic temperature” refers to a temperature that is about −50° C. or below.

A “diluent” is a gas used to lower the concentration of an oxidant fed to a gas turbine to combust a fuel, a gas used to lower the concentration of a fuel fed to a gas turbine that is combusted with an oxidant, a gas used to reduce the temperature of the products of combustion of a fuel and an oxidant fed to a gas turbine, or any combination thereof. The diluent may be an excess of nitrogen, carbon dioxide, combustion exhaust, or any number of other gases. In embodiments, the diluent may also provide cooling to a combustor.

“Enhanced oil recovery” or “EOR” refers to processes for enhancing the recovery of hydrocarbons from subterranean reservoirs by the introduction of materials not naturally occurring in the reservoir.

An “equivalence ratio” refers to the mass ratio of fuel to oxygen entering a combustor divided by the mass ratio of fuel to oxygen when the ratio is stoichiometric. A perfect combustion of fuel and oxygen to form carbon dioxide and water would have an equivalence ratio of 1. A too lean mixture, e.g., having more oxygen than fuel, would provide an equivalence ratio less than 1, while a too rich mixture, e.g., having more fuel than oxygen, would provide an equivalence ratio greater than 1.

A “fuel” includes any number of hydrocarbons that may be combusted with an oxidant to power a gas turbine. Such hydrocarbons may include natural gas, treated natural gas, kerosene, gasoline, or any number of other natural or synthetic hydrocarbons. In one embodiment, natural gas from an oil field is purified and used to power the turbine. In another embodiment, a reformed gas, for example, created by processing a hydrocarbon in a steam reforming process may be used to power the turbine.

The term “gas” is used interchangeably with “vapor,” and is defined as a substance or mixture of substances in the gaseous state as distinguished from the liquid or solid state. Likewise, the term “liquid” means a substance or mixture of substances in the liquid state as distinguished from the gas or solid state.

A “gas turbine engine” operates on the Brayton cycle. If the exhaust gas is vented to the atmosphere, this is termed an open Brayton cycle, while recycling of the exhaust gas gives a closed Brayton cycle. As used herein, a “gas turbine” typically includes a compressor section, a number of combustors, and an expander turbine section. The compressor may be used to compress an oxidant, which is mixed with a fuel and channeled to the combustors. The mixture of fuel and oxidant is then ignited to generate hot combustion gases. The combustion gases are channeled to the expander turbine section which extracts energy from the combustion gases for powering the compressor, as well as producing useful work to power a load. In embodiments discussed herein, the oxidant may be provided to the combustors by an external compressor, which may or may not be mechanically linked to the shaft of the gas turbine engine. Further, in embodiments, the compressor section may be used to compress a diluent, such as recycled exhaust gases, which may be fed to the combustors as a coolant.

A “heat exchanger” broadly means any device capable of transferring heat from one media to another media, including particularly any structure, e.g., device commonly referred to as a heat exchanger. Heat exchangers include “direct heat exchangers” and “indirect heat exchangers.” Thus, a heat exchanger may be a plate-and-frame, shell-and-tube, spiral, hairpin, core, core-and-kettle, double-pipe or any other type of known heat exchanger. “Heat exchanger” may also refer to any column, tower, unit or other arrangement adapted to allow the passage of one or more streams therethrough, and to affect direct or indirect heat exchange between one or more lines of refrigerant, and one or more feed streams.

A “heat recovery steam generator” or “HRSG” is a heat exchanger or boiler that recovers heat from a hot gas stream. It produces steam that can be used in a process or used to drive a steam turbine. A common application for an HRSG is in a combined-cycle power plant, where hot exhaust from a gas turbine is fed to the HRSG to generate steam which in turn drives a steam turbine. This combination produces electricity more efficiently than either the gas turbine or steam turbine alone.

A “hydrocarbon” is an organic compound that primarily includes the elements hydrogen and carbon, although nitrogen, sulfur, oxygen, metals, or any number of other elements may be present in small amounts. As used herein, hydrocarbons generally refer to components found in raw natural gas, such as CH₄, C₂H₂, C₂H₄, C₂H₆, C₃ isomers, C₄ isomers, benzene, and the like.

“Natural gas” refers to a multi-component gas obtained from a crude oil well or from a subterranean gas-bearing formation. The composition and pressure of natural gas can vary significantly. A typical natural gas stream contains methane (CH₄) as a major component, i.e., greater than 50 mol % of the natural gas stream is methane. The natural gas stream can also contain ethane (C₂H₆), higher molecular weight hydrocarbons (e.g., C₃-C₂₀ hydrocarbons), one or more acid gases (e.g., carbon dioxide or hydrogen sulfide), or any combinations thereof. The natural gas can also contain minor amounts of contaminants such as water, nitrogen, iron sulfide, wax, crude oil, or any combinations thereof. The natural gas stream may be substantially purified prior to use in embodiments, so as to remove compounds that may act as poisons.

An “oxidant” is a gas mixture that can be flowed into the combustors of a gas turbine engine to combust a fuel. As used herein, the oxidant may be oxygen mixed with any number of other gases as diluents, including carbon dioxide (CO₂), nitrogen (N₂), air, combustion exhaust, and the like. Other gases that function as oxidizers may be present in the oxidant mixture in addition to oxygen, including ozone, hydrogen peroxide, NOxs, and the like.

“Pressure” is the force exerted per unit area by the gas on the walls of the volume. Pressure can be shown as pounds per square inch (psi). “Atmospheric pressure” refers to the local pressure of the air. “Absolute pressure” (psia) refers to the sum of the atmospheric pressure (14.7 psia at standard conditions) plus the gage pressure (psig). “Gauge pressure” (psig) refers to the pressure measured by a gauge, which indicates only the pressure exceeding the local atmospheric pressure (i.e., a gauge pressure of 0 psig corresponds to an absolute pressure of 14.7 psia). The term “vapor pressure” has the usual thermodynamic meaning. For a pure component in an enclosed system at a given pressure, the component vapor pressure is essentially equal to the total pressure in the system.

“Substantial” when used in reference to a quantity or amount of a material, or a specific characteristic thereof, refers to an amount that is sufficient to provide an effect that the material or characteristic was intended to provide. The exact degree of deviation allowable may in some cases depend on the specific context.

Overview

Embodiments described herein provide a system and method for recovering CO₂ from a gas mixture via a CO₂ separation system that includes a rotating freezer/melter. More specifically, embodiments described herein provide a system and method for recovering CO₂ from an exhaust gas exiting a power plant or a natural gas stream including CO₂, for example. For example, in various embodiments, power is generated via a power plant, and CO₂ is recovered from the exhaust gas exiting the power plant. According to such embodiments, a gas mixture including CO₂, H₂O, and inert gas is generated by a power plant during the generation of power. A dehydration system dehydrates the gas mixture, and a CO₂ separation system recovers the CO₂ from the dehydrated gas mixture. Furthermore, according to embodiments described herein, the CO₂ separation system includes a rotating freezer/melter for recovering the CO₂ from the dehydrated gas mixture.

Systems for Power Generation and CO₂ Recovery

FIG. 1 is a block diagram of a system 100 for power generation and CO₂ recovery. In the system 100, oxidant 102 and fuel gas 104 are provided to a power plant 106, for example, a gas turbine generator (GTG), at a substantially stoichiometric ratio. The oxidant 102 can be air having about 78% N₂ and about 21% oxygen and, thus, the ratio would be calculated between the fuel gas 104 and the oxygen portion of the oxidant 102. The fuel gas 102 and oxygen are substantially completely combusted in the GTG of the power plant 106 to form an exhaust gas that includes CO₂, H₂O, and inert gas such as N₂. The exhaust gas may also include trace amounts of carbon monoxide (CO), nitrogen oxides (NOx), oxygen (O₂), and fuel. The energy from the exhaust gas is used to drive an expander turbine that turns a shaft. A generator coupled to the shaft generates electricity 108.

In some embodiments, the power plant 106 is a semi-closed Brayton cycle power plant. The power plant 106 may be a combined cycle power plant that includes both a semi-closed Brayton cycle and a Rankine cycle. In such embodiments, the exhaust stream from the expander turbine of the semi-closed Brayton cycle can be used to boil water or other heat transfer fluids in a heat recovery steam generator (HRSG) that can be used to power the Rankine cycle power plant. In the Rankine cycle power plant, the steam or other vapor can be used to drive a turbine and generate more electricity 108.

The treated stream from the power plant 106 forms a gas mixture 110. The gas mixture 110 may include primarily CO₂, H₂O, and inert gas. The gas mixture 110 is flowed through a dehydration system 112, in which the H₂O 114 is separated from the CO₂ and inert gas within the gas mixture 110.

The dehydrated gas mixture 116 is then flowed into a CO₂ separation system 118. Within the CO₂ separation system 118, the CO₂ 120 is separated from the inert gas 122 within the dehydrated gas mixture 116. In various embodiments, this is accomplished using a rotating freezer/melter within the CO₂ separation system 118, as discussed further with respect to FIGS. 3-10.

The block diagram of FIG. 1 is not intended to indicate that the system 100 is to include all of the components shown in FIG. 1. Moreover, the system 100 may include any number of additional components not shown in FIG. 1, depending on the details of the specific implementation. For example, in various embodiments, the gas mixture 110 is flowed through a precooler before being flowed into the dehydration system 112. The precooler may lower the temperature of the gas mixture 110 in preparation for the recovery of the CO₂ 120 from the gas mixture 110.

FIG. 2 is a process flow diagram of a combined cycle power plant 200 that can be used to produce electricity 202 and generate a diluent gas mixture including CO₂. In various embodiments, the combined cycle power plant 200 includes a semi-closed Brayton cycle including, for example, an expander turbine 206, and a Rankine cycle including, for example, a HRSG 208.

Within the combined cycle power plant 200, oxidant 210 and fuel gas 212 are fed to a combustor 214 to be burned. A compressed diluent stream 216 is also fed to the combustor 214 to lower the total amount of oxidant 201 and fuel gas 212 that is used, which allows the combustion process to be run at near stoichiometric conditions without overheating the combustor 214 or the expander turbine 206. As a result, the amount of O₂ and CO generated in the combustion process is decreased, and hot exhaust gas 218 exiting the combustor includes mostly CO₂, H₂O, and N₂, in addition to some trace gases, such as CO, H₂ and NOx.

The oxidant 210 and fuel gas 212 pressures may be increased, for example, using compressors, to boost the pressure to match the injection pressure of the compressed diluent stream 216 at the combustor 214. The hot exhaust gas 218 from the combustor 214 is flowed to the expander turbine 206, which uses the energy of the hot exhaust gas 218 to spin a shaft 220. The shaft 220 provides mechanical energy to a compressor, completing the Brayton cycle. The shaft 220 may also provide mechanical energy to an electric generator 222 to generate electricity 202. The electric generator 222 may be directly coupled to the shaft 220 from the expander turbine 206, or may be coupled to the shaft 220 by a gear box, clutch, or other device.

From the expander turbine 206, the hot exhaust gas 218 is flowed to the HRSG 208. The HRSG 208 may boil a water stream 224 with the energy from the hot exhaust gas 218 to generate steam 226. The steam 226 that is generated can be used to drive a steam turbine 228 and spin a shaft 230. After exiting the steam turbine 228, the resulting low pressure steam 232 can be cooled and condensed, and can be used as the water stream 224 to feed the HRSG 208.

The shaft 230 from the steam turbine 228 can provide mechanical energy to an electric generator 234 to generate the electricity 202, or may be used power other devices, such as compressors. The electric generator 234 may be directly coupled to the shaft 230 from the steam turbine 228, or may be coupled to the shaft 230 by a gear box, clutch, or other device. Further, in the embodiment shown in FIG. 2, the expander turbine 206 and the steam turbine 228 are coupled to separate electric generators 222 and 234. However, it is to be understood that the expander turbine 206 and the steam turbine 228 may also be coupled, directly or indirectly, to one common electric generator.

The hot gas stream 236 exiting the HRSG 208 is flowed to a cooler 238. The cooler 238 chills the hot gas stream 236, causing the water vapor formed in the combustion process to condense out, allowing its removal as a separate water stream 240. After removal of the water stream 240, the chilled gas mixture 242 is provided to a compressor 244 for recompression, prior to feeding the compressed diluent stream 216 to the combustor 214 to aid in cooling the combustor 214. The recycling of the hot gas stream 236 as the diluent stream 216 partially closes the Brayton cycle in the combined cycle power plant 200, resulting in a semi-closed Brayton cycle.

As the fuel gas 212 and the oxidant 210 are continuously being fed to the combined cycle power plant 200 to maintain the combustion, a portion 246 of the diluent stream 216 is continuously removed to maintain the mass balance in the semi-closed Brayton cycle. The diluent stream 216 may include CO₂, H₂O, and inert gas.

According to embodiments described herein, the diluent stream 216 exiting the combined cycle power plant 200 is flowed into a dehydration system, in which the H₂O is removed from the diluent stream 216. The dehydrated diluent stream 216 is then flowed into a CO₂ separation system. Within the CO₂ separation system, the CO₂ is recovered from the diluent stream 216 using a rotating freezer/melter. The process of recovering the CO₂ from the diluent stream 216 is discussed further with respect to FIGS. 3-10.

FIG. 3 is a process flow diagram of a system 300 for low emissions power generation and CO₂ recovery. The system 300 provides for low emissions power generation using a combined cycle power plant including a semi-closed Brayton cycle that utilizes a gas turbine engine 302 and a Rankine cycle that utilizes an HRSG 304. In addition, the system 300 provides for the recovery of CO₂ from exhaust gases exiting the combined cycle power plant.

As shown in FIG. 3, air 306 and fuel gas 308 are fed to a combustor 310 to be burned within the semi-closed Brayton cycle. While air 306 is used as the oxidant in the embodiment shown in FIG. 3, it is to be understood that any other suitable type of oxidant may also be used in conjunction with the system 300.

A compressed diluent stream 312 is also fed to the combustor 310 to lower the total amount of air 306 and fuel gas 308 that is utilized for the combustion process. This may allow the combustion process to be run at or near stoichiometric conditions without overheating. As a result, the amount of O₂ and CO generated in the combustion process is decreased, and hot exhaust gas 314 exiting the combustor includes mostly CO₂, H₂O, and inert gas such as N₂.

The air 306 and fuel gas 308 pressures may be increased, for example, using compressors, to boost the pressure to match the injection pressure of the compressed diluent stream 312 at the combustor 310. For example, according to the embodiment shown in FIG. 3, the air 306 is compressed within an air compressor 316. The air compressor 316 includes at least one compression stage, and may include intercoolers, knock out drums, and any other suitable equipment. The compressed air 306 is then fed into the combustor 310 to be burned.

The hot exhaust gas 314 from the combustor 310 is flowed to an expander turbine 322 of the gas turbine engine 302, which uses the energy of the hot exhaust gas 314 to spin a shaft 324. The shaft 324 provides mechanical energy to an electric generator 326 to generate electricity 328. The electric generator 326 may be directly coupled to the shaft 324 from the expander turbine 322, or may be coupled to the shaft 324 by a gear box, clutch, or other device.

From the expander turbine 322, the hot exhaust gas 314 is flowed to the HRSG 304 within the Rankine cycle of the combined cycle power plant. The HRSG 304 boils a water stream 330 to generate steam 332 with the energy from the hot exhaust gas 314. In various embodiments, the generated steam 332 is used to drive the steam turbine, which uses the energy of the steam 332 to spin a shaft. The shaft may provide mechanical energy to an electric generator to generate additional electricity.

The hot gas stream 334 exiting the HRSG 304 is flowed to an exhaust gas recirculation (EGR) blower 336. The EGR blower 336 compresses the hot gas stream 334 and feeds the resulting compressed gas stream 338 into an EGR cooler 340. The EGR cooler 340 chills the compressed gas stream 338, producing a diluent stream 342. Cooling the hot gas stream 334 may also condense out water, drying the diluent stream 342.

The diluent stream 342 is then fed into a compressor 344. The compressor 344 compresses the diluent stream 342, producing the compressed diluent stream 312. In the embodiment shown in FIG. 3, the compressor 344 is coupled to the shaft 324, and the mechanical energy provided by the spinning of the shaft 324 by the expander turbine 322 is used to drive the compressor 344.

From the compressor 344, the compressed diluent stream 312 is fed to the combustor 310 to aid in cooling the combustor 310. The recycling of the hot gas stream 334 as the compressed diluent stream 312 partially closes the Brayton cycle in the combined cycle power plant, resulting in the semi-closed Brayton cycle.

As the air 306 and the fuel gas 308 are continuously being fed to the combustor 310 to maintain the combustion process, at least a portion of the compressed diluent stream 312 is continuously removed. For example, a portion of the diluent stream 312 may be removed as a gas mixture 346 including primarily CO₂, H₂O, and inert gas.

In some embodiments, the gas mixture 346 may be extracted from the combustor 310 after it has been burned and used to drive the expander turbine 322. For example, the gas mixture 346 may be extracted from the expander turbine 322 at about 2206 kilopascals (kPa) and 427 degrees Celsius (° C.). The gas mixture 346 is then cooled using a purge cooler 348 and, optionally, used to generate steam 332 within the HRSG 304.

After the gas mixture 346 has been cooled within the purge cooler 348, the gas mixture 346 is fed into a dehydration system 350. Within the dehydration system 350, the gas mixture 346 is dehydrated to remove the H₂O 352. In various embodiments, the gas mixture 346 is dehydrated such that there is a very low amount of H₂O 352 remaining in the gas mixture 346. For example, the dew point of the resulting dehydrated gas mixture 354 may be less than about −70° C., or lower. The resulting dehydrated gas mixture 354 exiting the dehydration system 350 may be at about 2206 kPa and 49° C.

The dehydrated gas mixture 354 is flowed into a CO₂ separation system 356 for the recovery of the CO₂ 358 from the dehydrated gas mixture 354. Specifically, the dehydrated gas mixture 354 is flowed into a heat exchanger 360 within the CO₂ separation system 356. Within the heat exchanger 360, the dehydrated gas mixture 354 is cooled to about −68° C. via indirect heat exchange with a low-temperature inert gas stream 362.

From the heat exchanger 360, the resulting low-temperature gas mixture 364 is flowed through a cryogenic expander 366. The cryogenic expander 366 lowers the pressure and temperature of the low-temperature gas mixture 364 to about 138 kPa and −101° C., respectively. At this condition, a portion of the CO₂ within the gas mixture 364 freezes to pure solid CO₂, resulting in the generation of a multiphase stream 368 including solid CO₂ and inert gas including some amount of residual CO₂.

The multiphase stream 368 is flowed into a rotating freezer/melter 370 including a freezing zone 372, a melting zone 374, and a rotor (not shown) that rotates throughout both the freezing zone 372 and the melting zone 374. Specifically, the multiphase stream 368 is flowed into the freezing zone 372 of the rotating freezer/melter 370. The rotor provides a porous media upon which the solid CO₂ within the multiphase stream 368 crystallizes and accumulates. The rotor also allows the inert gas including the residual CO₂ to pass through the porous media and exit the rotating freezer/melter as the low-temperature inert gas stream 362 at about 138 kPa and −101° C.

As the rotor rotates through the rotating freezer/melter 370, the solid CO₂ that has accumulated on the rotor enters the melting zone 374 of the rotating freezer/melter 370. Within the melting zone 374 of the rotating freezer/melter 370, the solid CO₂ is melted via contact with a high-pressure, high-temperature CO₂ stream 376 flowing though the melting zone 374. The resulting liquid CO₂ 378 flows through the rotor and exits the rotating freezer/melter 370 at about 1,034 kPa and −44° C.

The liquid CO₂ 378 is pumped to a pressure and temperature of about 13,790 kPa and −39° C. via a pump 380. The high-pressure liquid CO₂ 382 is converted to a vapor CO₂ stream via a refrigeration load 386 of about 33 MBTU/hr. The refrigeration load 386 may be internal or external to the combined cycle power plant and the CO₂ recovery system 356. In some embodiments, if the refrigeration load 386 is internal to the combined cycle power plant or the CO₂ recovery system 356, the refrigeration load 386 can be used to chill cooling water for the EGR cooler 340. In other embodiments, if the refrigeration load 386 is external to the combined cycle power plant and the CO₂ recovery system 356, the refrigeration load 386 can be used to enhance the recovery of natural gas liquids from a hydrocarbon reservoir, for example. The vapor CO₂ stream is then flowed out of the system 300 as the final CO₂ product 358. The final CO₂ product 358 may be used for EOR operations, or the CO₂ may be sequestered in a carbon storage system, such as a subterranean saline aquifer or depleted oil or gas reservoir, for example.

The inert gas stream 362 exiting the freezing zone 372 of the rotating freezer/melter 370 may include about 93.8% nitrogen, 5.0% carbon dioxide, and 1.12% argon, for example. The inert gas stream 362 is used to cool the dehydrated gas mixture 354 within the heat exchanger 360, resulting in the generation of a high-temperature inert gas stream 388 at about 103 kPa and 35° C.

After removal of most the CO₂ from the multiphase stream on the melting zone 374 of the rotating freezer/melter 370, the resulting high-temperature inert gas stream 388 is flowed into a CO₂ separation device 390. The CO₂ separation device 390 separates any remaining carbon dioxide from the nitrogen and argon within the high-temperature inert gas stream 388, resulting the generation of a vent gas stream 392 and a vapor CO₂ stream 394 at about 103 kPa and 38° C. In various embodiments, the CO₂ separation device 390 separates the carbon dioxide from the nitrogen and argon via an amine separation process, a potassium carbonate separation process, or any other suitable type of separation process.

The vapor CO₂ stream 394 is compressed within a compressor 396, producing the high-pressure, high-temperature CO₂ stream 376 at about 1,034 kPa and 38° C. The high-pressure, high-temperature CO₂ stream 376 is then flowed through the melting zone 374 of the rotating freezer/melter 370 and is used to melt the solid CO₂ that has accumulated on the rotor.

In some embodiments, the rotating freezer/melter 370 includes an additional zone for melting and removing accumulated water-ice that may result from inadequate dehydration of the gas mixture 354. For example, a portion of the vent gas stream 392 may be used to melt any accumulated water-ice within the additional zone. This de-icing procedure may be performed continuously or intermittently, depending on the details of the specific implementation.

Tables 1A and 1B list the properties of the streams flowing through various components of the system 300 of FIG. 3. However, it is to be understood that the streams flowing through the components of the system 300 of FIG. 3 are not limited to the properties shown in Tables 1A and 1B. Rather, the properties shown in Tables 1A and 1B merely represent one exemplary embodiment of the operation of the system 300 of FIG. 3.

TABLE 1A Properties of Streams Flowing through Various Components of FIG. 3 Component Number 354 358 362 364 366 368 368 376 Phase Vapor Super Critical Vapor Vapor Vapor Solid Vapor Mole flow rate (kmol/sec) 10.76 1.17 9.98 10.76 9.98 0.79 0.39 Temperature (deg C.) 48.89 37.78 −101.31 −67.78 −101.31 −101.31 37.78 Pressure (kPa) 2206 13720 138 2172 138 138 1103 External Power Added (MW) −26.68 External Heat Added (MW) Composition (mole fraction) Water 0.000 0.000 0.000 0.000 0.000 0.000 0.000 Nitrogen 0.870 0.000 0.938 0.870 0.938 0.000 0.000 CO2 0.119 1.000 0.050 0.119 0.050 1.000 1.000 Argon 0.010 0.000 0.011 0.010 0.011 0.000 0.000 Carbon Monoxide 0.001 0.000 0.001 0.001 0.001 0.000 0.000 Total 1.000 1.000 1.000 1.000 1.000 1.000 1.000

TABLE 1B Properties of Streams Flowing through Various Components of FIG. 3. Component Number 378 380 382 386 388 392 394 396 Phase Liquid Super Critical Vapor Vapor Vapor Mole flow rate (kmol/sec) 1.17 1.17 9.98 9.59 0.39 Temperature (deg C.) −44.38 −39.63 35.35 37.78 37.78 Pressure (kPa) 1034 13789 103 103 103 External Power Added (MW) 0.85 3.78 External Heat Added (MW) 9.80 −3.92 Composition (mole fraction) Water 0.000 0.000 0.000 0.000 0.000 Nitrogen 0.000 0.000 0.938 0.976 0.000 CO2 1.000 1.000 0.050 0.012 1.000 Argon 0.000 0.000 0.011 0.012 0.000 Carbon Monoxide 0.000 0.000 0.001 0.001 0.000 Total 1.000 1.000 1.000 1.000 1.000

The process flow diagram of FIG. 3 is not intended to indicate that the system 300 is to include all of the components shown in FIG. 3. Moreover, the system 300 may include any number of additional components not shown in FIG. 3, depending on the details of the specific implementation.

It is to be understood that any number of alternatives to the rotating freezer/melter 370 may be used according to embodiments described herein. For example, the porous media of the rotating freezer/melter 370 may be arranged on a linear conveyor belt or similar device to pass the media successively through freezing and melting zones in a similar manner as described herein. In addition, similar functionality may be achieved by the use of a number of vessels that contain similar porous media that may be sequenced by the action of valves or similar devices from a freezing mode to a melting mode.

FIG. 4 is a process flow diagram of another system 400 for low emissions power generation and CO₂ recovery. Like numbered items are as described with respect to FIG. 3. The system 400 of FIG. 4 is similar to the system 300 of FIG. 3. However, the CO₂ separation system 402 of the system 400 of FIG. 4 does not include the CO₂ separation device 390 that is included within the CO₂ separation system 356 of the system 300 of FIG. 3. Therefore, the system 400 of FIG. 4 may not recover as much CO₂ from the gas mixture 346 exiting the combined cycle power plant as the system 300 of FIG. 3. For example, the system 300 of FIG. 3 may recover over 60% of the CO₂ from the gas mixture 346, while the system 400 of FIG. 4 may recover only about 60% or less of the CO₂ from the gas mixture 346.

As discussed with respect to the system 300 of FIG. 3, the inert gas stream 362 exiting the freezing zone 372 of the rotating freezer/melter 370 is used to cool the dehydrated gas mixture 354 within the heat exchanger 360, resulting in the generation of the high-temperature inert gas stream at about 103 kPa and 35° C. However, in contrast to the system 300 of FIG. 3, the high-temperature inert gas stream within the system 400 of FIG. 4 is not flowed to the CO₂ separation device 390 discussed with respect to FIG. 3. Rather, the high-temperature inert gas stream exiting the heat exchanger 360 is flowed out of the system 400 as a vent gas stream 404.

Furthermore, as discussed with respect to the system 300 of FIG. 3, the liquid CO₂ 378 exits the rotating freezer/melter 370 at about 1034 kPa and −44° C. However, instead of pumping all of the liquid CO₂ 378 out of the system 400 as the final CO₂ product 358, a portion 406, e.g., about 50%, of the liquid CO₂ 378 is removed upstream of the pump 380. The remaining portion of the liquid CO₂ 378 is then pumped to a pressure and temperature of about 13,790 kPa and −39° C. via the pump 380. The high-pressure liquid CO₂ 382 is converted to a vapor CO₂ stream via a refrigeration load 386 of about 33 MBTU/hr. The vapor CO₂ stream is then flowed out of the system 300 as the final CO₂ product 358.

The portion 406 of the liquid CO₂ 378 that is removed upstream of the pump 380 is flowed to a second pump 408. The second pump 408 increases the pressure of the liquid CO₂ 378 to about 1,103 kPa, generating a vapor CO₂ stream 410. The temperature of the vapor CO₂ stream 410 is increased to about 38° C. via a refrigeration load 412 of about 24 MBTU/hr. The resulting high-pressure, high-temperature CO₂ stream 414 is then flowed through the melting zone 374 of the rotating freezer/melter 370 and is used to melt the solid CO₂ that has accumulated on the rotor.

The process flow diagram of FIG. 4 is not intended to indicate that the system 400 is to include all of the components shown in FIG. 4. Moreover, the system 400 may include any number of additional components not shown in FIG. 4, depending on the details of the specific implementation. Further, it can be noted that the system described herein is not limited to using a combined cycle power plant, but may also be used with the exhaust from Rankine power plants, or other sources of CO₂ contaminated gases, such as high CO₂ content natural gas.

Rotating Freezer/Melter for CO₂ Recovery

FIG. 5 is a perspective view of a rotating freezer/melter 500 that may be used to recover CO₂ from a gas mixture. The rotating freezer/melter 500 of FIG. 5 may be used as the rotating freezer/melter 370 within the systems 300 and 400 of FIGS. 3 and 4.

The rotating freezer/melter 500 includes a freezing zone 502, a melting zone 504, and a rotor 506. The rotor 506 may continuously rotate through both the freezing zone 502 and the melting zone 504 of the rotating freezer/melter 500, as indicated by arrow 508. The rotor 506 may be constructed of crinkle wire mesh, packing, porous ceramic, or any other suitable porous material that provides enough surface area for solid CO₂ to accumulate on the rotor 506 without blocking the flow of gases or liquids through the rotor 506.

A multiphase stream 510 including solid CO₂ flows into the freezing zone 502 of the rotating freezer/melter 500. The solid CO₂ within the multiphase stream 510 crystallizes and accumulates on the rotor 506, while the inert gas flows through the rotor 506 and exits as an inert gas stream 512 including residual CO₂.

As the rotor rotates through the rotating freezer/melter 500, the solid CO₂ that has accumulated on the rotor 506 passes through the melting zone 504 of the rotating freezer/melter 500. Within the melting zone 504 of the rotating freezer/melter 500, the solid CO₂ comes in contact with a high-pressure, high-temperature CO₂ stream 514 flowing through the melting zone 504. As a result, the solid CO₂ is melted, and the high-pressure, high-temperature CO₂ stream is condensed, forming a combined liquid CO₂ stream 516. The liquid CO₂ stream 516 flows through the rotor 506 and out of the rotating freezer/melter 500 as the recovered CO₂ product.

FIG. 5 is not intended to indicate that the rotating freezer/melter 500 is to include all of the components shown in FIG. 5. Moreover, the rotating freezer/melter 500 may include any number of additional components not shown in FIG. 5, depending on the details of the specific implementation.

FIG. 6 is a cross-sectional view of the rotating freezer/melter 500 that may be used to recover CO₂ from a gas mixture. Like numbered items are as described with respect to FIG. 5. As shown in FIG. 6, the rotor 506 rotates about an axis 600 that extends through the center of the rotating freezer/melter 500.

According to embodiments described herein, only the solid CO₂ that has accumulated on the rotor 506 is to be allowed to pass directly from the freezing zone 502 to the melting zone 504 of the rotating freezer/melter 500. Thus, in various embodiments, a brush seal 602 or other sealing device is used to individually seal both the freezing zone 502 and the melting zone 504 of the rotating freezer/melter 500. Sealing both the freezing zone 502 and the melting zone 504 of the rotating freezer/melter 500 prevents gases or liquids from flowing from the freezing zone 502 to the melting zone 504, or vice versa. U.S Patent Application Publication No. 2008/0251234 by Wilson et al. and U.S. Patent Application Publication No. 2009/0000762 by Wilson et al. describe a rotary air-preheater using brush seals and other sealing improvements that may be adapted to seal the freezing and melting zones 502 and 504 of the rotating freezer/melter 500.

Furthermore, the freezing zone 502 and the melting zone 504 may include separate inlets and outlets to allow gases or liquids to flow into and out of the two zones 502 and 504 of the rotating freezer/melter 500 without mixing. Specifically, the freezing zone 502 includes a freezer inlet 604 and a freezer outlet 606. The multiphase stream 510 may flow into the freezing zone 502 via the freezer inlet 604, and the inert gas stream 512 may flow out of the freezing zone 502 via the freezer outlet 606. The melting zone 504 includes a melter inlet 608 and a melter outlet 610. The high-pressure, high-temperature CO₂ stream 514 flows into the melting zone 504 via the melter inlet 608, and the liquid CO₂ stream 516 flows out of the melting zone 504 via the melter outlet 610. Therefore, the freezing zone 502 and the melting zone 504 of the rotating freezer/melter 500 include separate flow paths that are only connected via the rotor 506.

FIG. 6 is not intended to indicate that the rotating freezer/melter 500 is to include all of the components shown in FIG. 6. Moreover, the rotating freezer/melter 500 may include any number of additional components not shown in FIG. 6, depending on the details of the specific implementation.

FIG. 7 is a perspective view of the rotor 506 of the rotating freezer/melter 500 of FIGS. 5 and 6. As shown in FIG. 7, the rotor 506 may be constructed of a material including a number of pores 700, such as porous ceramic, for example. The pores 700 may provide enough surface area for solid CO₂ to accumulate on the rotor 506 without blocking the flow of gases or liquids through the rotor 506. The flow path for the flow of gases or liquids through the rotor 506 may vary depending on specific conditions, as discussed further with respect to FIG. 8.

FIG. 7 is not intended to indicate that the rotor 700 is to include all of the components shown in FIG. 7. Moreover, the rotor 700 may include any number of additional components not shown in FIG. 7, depending on the details of the specific implementation.

FIG. 8 is a schematic showing flow paths 800 within a section 802 of the rotor 506 of FIG. 7. As shown in FIG. 8, the pores 700 within the rotor 506 may cause the flow path 800 for a substance passing through the rotor 506 to be tortuous rather than straight. Moreover, the flow path 800 may vary depending on specific conditions. In particular, the flow path 800 of a substance passing through the rotor 506 may depend at least in part on whether the pores are evenly or unevenly spaced, and whether the pores 700 are of a uniform size or differing sizes. In addition, the flow path 800 for a substance passing through the rotor 506 may depend on whether the substance is in the gas phase or the liquid phase, as well as the pressure and temperature of the substance.

The schematic of FIG. 8 is not intended to indicate that the gas flow paths 800 within the rotor 700 are to be exactly as shown in FIG. 8. Rather, the gas flow paths 800 within the rotor 700 may include any suitable variation of those shown in FIG. 7, depending on the details of the specific implementation.

FIG. 9 is a perspective view of another rotor 900 that may be used for the rotating freezer/melter 500 of FIGS. 5 and 6. As shown in FIG. 9, the rotor 900 includes a number of layers 902 of metal mesh screens 904. Each metal mesh screen 904 may include a number of small holes 906. The holes 906 may provide enough surface area for solid CO₂ to accumulate on the rotor 900 without blocking the flow of the gases or liquids through the rotor 900. The flow path for the flow of gases or liquids through the rotor 900 may vary depending on conditions that are similar to those discussed with respect to FIG. 8.

FIG. 9 is not intended to indicate that the rotor 900 is to include all of the components shown in FIG. 9. Moreover, the rotor 900 may include any number of additional components not shown in FIG. 9, depending on the details of the specific implementation.

Method for Power Generation and CO₂ Recovery

FIG. 10 is a process flow diagram of a method 1000 for power generation and CO₂ recovery. The method 1000 may be implemented by any of the systems 100-400 described with respect to FIGS. 1-4. The method 1000 may also be implemented by any variation of the systems 100-400 described with respect to FIGS. 1-4, or any suitable alternative system that is capable of integrating power generation with CO₂ recovery. Furthermore, in various embodiments, the rotating freezer/melter 500 discussed with respect to FIGS. 5-9 may be used to implement the method 1000.

The method 1000 begins at block 1002, at which power is produced via a power plant. An exhaust gas from the power plant provides a gas mixture including CO₂, H₂O, and inert gas. The inert gas may include nitrogen, argon, and any number of other trace gases.

In various embodiments, producing power via the power plant includes providing mechanical energy via an expander turbine of a gas turbine engine using energy extracted from the gas mixture after combustion of the gas mixture in a combustor and generating electricity via a generator using the mechanical energy provided by the expander turbine. Further, in various embodiments, producing power via the power plant also includes generating steam via a HRSG by heating a boiler with an exhaust stream from the expander turbine, providing mechanical energy via a steam turbine using energy extracted from the steam generated by the HRSG, and generating electricity via a generator using the mechanical energy provided by the steam turbine. In some embodiments, one common generator is used to generate electricity from the mechanical energy provided by the expander turbine and the steam turbine, while, in other embodiments, separate generators are used.

At block 1004, the CO₂ is recovered from the gas mixture via a CO₂ separation system including a rotating freezer/melter. This may be accomplished by capturing solid CO₂ on a rotor of the rotating freezer/melter while the rotor is in a freezing zone of the rotating freezer/melter and flowing an inert gas stream through the rotor while the rotor is in the freezing zone. The solid CO₂ that is captured on the rotor may be melted to form liquid CO₂ while the rotor is in a melting zone of the rotating freezer/melter, and the liquid CO₂ may be flowed through the rotor while the rotor is in the melting zone.

In some embodiments, a portion of the liquid CO₂ is recycled to the melting zone of the rotating freezer/melter and is used to melt the solid CO₂ within the melting zone. In addition, in some embodiments, residual CO₂ is recovered from the inert gas stream exiting the rotating melter/freezer via a CO₂ separation device downstream of the freezing zone of the rotating freezer/melter. The recovered CO₂ may be pressurized via a compressor to produce a pressurized CO₂ vapor stream, and the pressurized CO₂ vapor stream may be used to melt the solid CO₂ within the melting zone of the rotating freezer/melter.

Furthermore, in some embodiments, the H₂O is removed from the gas mixture via a dehydration system upstream of the CO₂ separation system. In addition, the solid CO₂ may be formed from the CO₂ within the gas mixture using a heat exchanger and an expander upstream of the rotating freezer/melter.

The process flow diagram of FIG. 10 is not intended to indicate that the steps of the method 1000 are to be executed in any particular order, or that all of the steps of the method 1000 are to be included in every case. Further, any number of additional steps may be included within the method 1000, depending on the details of the specific implementation.

FIG. 11 is a generalized process flow diagram of a method 1100 for recovering CO₂ from a gas mixture. The method 1100 may be used to recover CO₂ from any gas mixture including a substantial amount of CO₂. For example, in some embodiments, the method 1100 is used to remove CO₂ from an exhaust gas exiting a power plant. In such embodiments, the method 1000 may be implemented by any of the systems 100-400 described with respect to FIGS. 1-4, for example. In other embodiments, the method 1100 is used to remove CO₂ from a natural gas stream including a substantial amount of CO₂. In such embodiments, the method 1100 may be implemented by the system 1200 discussed with respect to FIG. 12. Furthermore, in various embodiments, the rotating freezer/melter 500 discussed with respect to FIGS. 5-9 may be used to implement the method 1100.

The method 1100 begins at block 1102, at which a gas mixture including CO₂ is obtained. The gas mixture may also include any number of other gaseous components. For example, the gas mixture may be an exhaust gas including CO₂, nitrogen, and any number of other inert gases, or the gas mixture may be a natural gas stream including natural gas, CO₂, and any number of other residual gases.

At block 1104, the CO₂ is recovered from the gas mixture via a CO₂ separation system including a rotating freezer/melter. This may be accomplished by capturing solid CO₂ on a rotor of the rotating freezer/melter while the rotor is in a freezing zone of the rotating freezer/melter and flowing the gas mixture through the rotor while the rotor is in the freezing zone. The solid CO₂ that is captured on the rotor may be melted to form liquid CO₂ while the rotor is in a melting zone of the rotating freezer/melter. The liquid CO₂ may then be flowed through the rotor while the rotor is in the melting zone and recovered as the CO₂ product.

The process flow diagram of FIG. 11 is not intended to indicate that the steps of the method 1100 are to be executed in any particular order, or that all of the steps of the method 1100 are to be included in every case. Further, any number of additional steps may be included within the method 1100, depending on the details of the specific implementation.

System for Recovering CO, from Natural Gas

FIG. 12 is a block diagram of a system 1200 for recovering CO₂ from natural gas. In the system 1200, a high CO₂ natural gas stream 1202 is obtained from a high CO₂ natural gas field 1204. The high CO₂ natural gas stream 1202 is flowed through a dehydration system 1206. Within the dehydration system 1206, H₂O 1208 is separated from the high CO₂ natural gas stream 1202, producing a dehydrated high CO₂ natural gas stream 1210.

The dehydrated high CO₂ natural gas stream 1210 is then flowed into a CO₂ separation system 1212. Within the CO₂ separation system 1212, CO₂ is separated from the dehydrated high CO₂ natural gas stream 1216, producing a purified natural gas stream 1214 and a CO₂ product stream 1216. In various embodiments, this is accomplished using a rotating freezer/melter within the CO₂ separation system 1212, such as the rotating freezer/melter 500 discussed with respect to FIGS. 5-9.

The block diagram of FIG. 12 is not intended to indicate that the system 1200 is to include all of the components shown in FIG. 12. Moreover, the system 1200 may include any number of additional components not shown in FIG. 12, depending on the details of the specific implementation.

While the present techniques may be susceptible to various modifications and alternative forms, the exemplary embodiments discussed herein have been shown only by way of example. However, it should again be understood that the techniques is not intended to be limited to the particular embodiments disclosed herein. Indeed, the present techniques include all alternatives, modifications, and equivalents falling within the true spirit and scope of the appended claims. 

What is claimed is:
 1. A system for recovering carbon dioxide (CO₂), comprising a CO₂ separation system configured to recover the CO₂ from a gas mixture, wherein the CO₂ separation system comprises a rotating freezer/melter.
 2. The system of claim 1, wherein the rotating freezer/melter comprises a freezing zone, a melting zone, and a rotor.
 3. The system of claim 2, wherein solid CO₂ formed from the CO₂ within the gas mixture is captured on the rotor within the freezing zone, and wherein the gas mixture passes through the rotor within the freezing zone.
 4. The system of claim 3, wherein the solid CO₂ is formed from the CO₂ within the gas mixture via a heat exchanger and an expander upstream of the rotating freezer/melter.
 5. The system of claim 3, wherein the solid CO₂ melts and passes through the rotor as liquid CO₂ within the melting zone.
 6. The system of claim 5, wherein a portion of the liquid CO₂ is recycled to the melting zone of the rotating freezer/melter and used to melt the solid CO₂ within the melting zone.
 7. The system of claim 3, wherein the CO₂ separation system comprises a CO₂ separation device downstream of the freezing zone of the rotating freezer/melter, and wherein the CO₂ separation device recovers residual CO₂ from the gas mixture.
 8. The system of claim 7, wherein the CO₂ separation device recovers the residual CO₂ from the gas mixture via an amine separation process.
 9. The system of claim 7, wherein the CO₂ separation device recovers the residual CO₂ from the gas mixture via a potassium carbonate separation process.
 10. The system of claim 7, wherein the residual CO₂ recovered via the CO₂ separation device is pressurized via a compressor to produce a pressurized CO₂ vapor stream, and wherein the pressurized CO₂ vapor stream is used to melt the solid CO₂ within the melting zone of the rotating freezer/melter.
 11. The system of claim 1, comprising a power plant configured to generate power, wherein an exhaust gas from the power plant provides the gas mixture, and wherein the gas mixture comprises the CO₂, H₂O, and inert gas.
 12. The system of claim 11, comprising a dehydration system configured to remove the H₂O from the gas mixture.
 13. The system of claim 11, wherein the power plant comprises: an expander turbine configured to provide mechanical energy by extracting energy from the gas mixture after combustion of the gas mixture in a combustor; and a generator configured to generate electricity from the mechanical energy provided by the expander turbine.
 14. The system of claim 11, wherein the power plant comprises a combined cycle power plant.
 15. The system of claim 14, wherein the combined cycle power plant comprises: an expander turbine configured to provide mechanical energy by extracting energy from the gas mixture after combustion of the gas mixture in a combustor; a heat recovery steam generator (HRSG) configured to generate steam by heating a boiler with an exhaust stream from the expander turbine; a steam turbine configured to provide mechanical energy by extracting energy from the steam generated by the HRSG; and a generator configured to generate electricity from the mechanical energy provided by the expander turbine and the steam turbine.
 16. The system of claim 11, wherein a portion of the gas mixture is recycled to the power plant.
 17. The system of claim 1, wherein the gas mixture comprises a natural gas stream comprising the CO₂.
 18. A method for recovering carbon dioxide (CO₂), comprising recovering the CO₂ from a gas mixture comprising the CO₂ via a CO₂ separation system, wherein the CO₂ separation system comprises a rotating freezer/melter.
 19. The method of claim 18, wherein recovering the CO₂ from the gas mixture via a CO₂ separation system comprises: capturing solid CO₂ formed from the gas mixture on a rotor of the rotating freezer/melter while the rotor is in a freezing zone of the rotating freezer/melter; flowing the gas mixture through the rotor while the rotor is in the freezing zone; melting the solid CO₂ that is captured on the rotor to form liquid CO₂ while the rotor is in a melting zone of the rotating freezer/melter; and flowing the liquid CO₂ through the rotor while the rotor is in the melting zone.
 20. The method of claim 19, comprising forming the solid CO₂ from the CO₂ within the gas mixture using a heat exchanger and an expander upstream of the rotating freezer/melter.
 21. The method of claim 19, comprising: recycling a portion of the liquid CO₂ to the melting zone of the rotating freezer/melter; and using the portion of the liquid CO₂ to melt the solid CO₂ within the melting zone.
 22. The method of claim 19, comprising: recovering residual CO₂ from the gas mixture via a CO₂ separation device downstream of the freezing zone of the rotating freezer/melter; pressurizing the recovered residual CO₂ via a compressor to produce a pressurized CO₂ vapor stream; and using the pressurized CO₂ vapor stream to melt the solid CO₂ within the melting zone of the rotating freezer/melter.
 23. The method of claim 18, comprising producing power via a power plant, wherein an exhaust gas from the power plant provides the gas mixture, and wherein the gas mixture comprises the CO₂, H₂O, and inert gas.
 24. The method of claim 23, comprising removing the H₂O from the gas mixture via a dehydration system.
 25. The method of claim 23, wherein producing the power via the power plant comprises: providing mechanical energy via an expander turbine using energy extracted from the gas mixture after combustion of the gas mixture in a combustor; and generating electricity via a generator using the mechanical energy provided by the expander turbine.
 26. The method of claim 23, wherein producing the power via the power plant comprises: providing mechanical energy via an expander turbine using energy extracted from the gas mixture after combustion of the gas mixture in a combustor; generating steam via a heat recovery steam generator (HRSG) by heating a boiler with an exhaust stream from the expander turbine; providing mechanical energy via a steam turbine using energy extracted from the steam generated by the HRSG; and generating electricity via a generator using the mechanical energy provided by the expander turbine and the steam turbine.
 27. A rotating freezer/melter for recovering carbon dioxide (CO₂) from a gas mixture, comprising: a freezing zone; a melting zone; and a rotor configured to rotate through the freezing zone and the melting zone; wherein solid CO₂ formed from a gas mixture is captured on the rotor while the rotor is rotating through the freezing zone, and wherein the solid CO₂ melts and flows through the rotor as liquid CO₂ while the rotor is rotating through the melting zone.
 28. The rotating freezer/melter of claim 27, wherein the rotor comprises porous ceramic.
 29. The rotating freezer/melter of claim 27, wherein the rotor comprises a plurality of layers of metal mesh screens.
 30. The rotating freezer/melter of claim 27, comprising a sealing device for individually sealing both the freezing zone and the melting zone of the rotating freezer/melter.
 31. The rotating freezer/melter of claim 27, wherein gaseous components within the gas mixture flow through the rotor while the rotor is rotating through the freezing zone. 